Intermittent generators, such as solar- and wind-based generators, only produce electricity when their energy source is available (e.g. when the sun shines or the wind blows). Conversely, when their energy source is not available, intermittent generators do not produce power (and may even consume small amounts of electricity for their power electronics).

With distributed generation (DG), intermittent generators are often connected behind the same utility meter as customer loads (e.g. HVAC, consumer electronics, lights) that can be turned off and on based on the needs of the customer. As a result, on-site electric power production does not necessarily match on-site electric power consumption (also known as the customer 'load' or 'demand') on a moment-to-moment basis.

Therefore, sometimes DG customers are importing power from the grid (consuming more power than they are generating on-site), and other times DG customers are exporting power to the grid (generating more power than they are consuming on-site). In Massachusetts, net metering allows these DG customers to financially balance out the total amount of energy imported with the total amount of energy exported over the course of a billing period (typically about a month). Then, the customer is only billed (or credited) for the net difference between these two amounts.

In accordance with Massachusetts state law (as clarified in Massachusetts DPU ruling 11-11-A, issued 10/25/2012), the
Massachusetts System of Assurance (MassACA) has been implemented by the DPU to monitor the available cap allocations and determine eligibility for Net Metering with regard to the private and public caps.

Note: Distributed generation facilities, which are not eligible for Net Metering Services, may alternatively register as Qualifying Facilities in order to sell excess power to National Grid at wholesale rates. (Refer to the section about Qualifying Facilities below.)

Net Metering Cap Information:
Effective November 4, 2012, the net metering provision as approved by the Massachusetts DPU limits net metering to 4% of a utility's historical peak load for private customers and 5% for public entities.
National Grid's Massachusetts Electric historical peak load of 5,131 MWs occurred on August 2, 2006; making the private customer limit equal to 205.24 MW and the public customer limit equal to 256.55. MW.
National Grid's Nantucket Electric historical peak load of 45.47 MW occurred on July 19, 2013; making the private customer limit equal to 1.818 MW and the public customer limit equal to 2.273 MW.

Note: The private limit also includes those public entities whose generator rating is less than 60kW. Neither limit includes projects that qualify as exempt Class I Net Metering Facilities (see 220 CMR 18.07(5)).

The aggregate capacities of Net Metering projects that fall under these caps (as of February 20, 2015) are as follows:

Net Metering Service
(already connected)

Mass. Electric Territory

Nantucket Electric Territory

Private Cap*

120,099 kW

364 kW

Public Cap*

142,454 kW

0 kW

*In order to receive Net Metering Services, all interconnection projects that larger than 10kW on a single-phase service (or larger than 25kW on a three-phase service) must apply for and receive a cap allocation through
MassACA.

Net Metering in Practice:

In most cases, National Grid simply reads the retail meter at the beginning and end of the billing period to determine the net energy import or export (in kWh) for the billing period.

If the DG customer is a net importer (using more energy on-site than it generates) during the billing period, then National Grid will simply bill the DG customer for the net energy use (in kWh) during the billing period (which has already been reduced due to the on-site generation).

If the DG customer is a net exporter (generating more energy on-site than it uses) during the billing period, then National Grid will: (1) bill the DG customer for zero kWh of energy usage and (2) credit the DG customer for the net energy exported to the grid during the billing period in the form of Net Metering Credits (NMC). Refer to the "Calculating Net Metering Credits" and "Allocating Net Metering Credits" sections below for more details.

In all cases, DG customers will still be responsible to pay for any demand charge (measured in kW) and/or customer charges associated with the account. These charges will not be credited back to DG customers.

Each billing period, National Grid calculates a host customer's NMC by adding up the applicable charges (in $/kWh) and multiplying the total by the net exported energy (in kWh). The host customerís bill is credited with its NMC as a dollar-value credit. Refer to the "Allocating Net Metering Credits" section below for more details.

Generally, the current values for each charge can be found on the most recent electric bill for the host account. However, more information about rates and prices can be found using the following links:

Note: Customers are always responsible for the customer charge and any demand related charges.

Allocating Net Metering Credits (NMC):

Once the Net Metering Credits (NMC) are calculated and credited to a host customer's account, the host customer can allocate the NMC to other National Grid electric accounts using the
Schedule Z form.

Any NMC that are allocated to other National Grid electric accounts will appear as a dollar-value credit on the recipient's next bill. Any NMC that are not allocated to other accounts will accrue on the host customer's account as a dollar-value credit. Generally, unused NMC will accrue on accounts indefinitely (or until they are used). In other words, customers with Net Metering Services should NOT expect to receive a check or payment as compensation for accrued NMC.

There are some limitations on allocating NMC. National Grid's Massachusetts Electric customers may allocate NMC to any other Massachusetts Electric account within the same New England ISO load zone (e.g. Western and Central MA zone; Boston & Northeastern MA zone; or Southeastern MA zone). National Grid's Nantucket Electric customers may only allocate NMC to other Nantucket Electric accounts. Also, a "Public Net Metering Facility" may only allocate NMC to the electric accounts of other public entities (with the approval of the Massachusetts DPU).

Note: After a Net Metered Facility comes online, the NMC may be earned but not be applied to the host customer's account until the following billing period. If the host customer is transferring NMC to other accounts, it may take one additional billing period before the NMC are applied to the recipient account(s).

Qualifying Facilities (for FERC):

Qualifying Facilities (QF) have several options for metering and billing. For a description of qualifying facilities please see the
Electric-Qualifying Facilities webpage on the Federal Energy Regulatory Commission's website.

For more information about the options for Qualifying Facilities, please review the
Massachusetts DPU QF website, and then view our
Rates Tariff (pdf) and refer to "Qualifying Facility Power Purchase Rate P" (the second to last item in the document).